Did you buy a wrist monitor? Those aren't as accurate as arm cuffs, but if you did buy one, it's not a big deal.

You should be able to use an arm cuff on either arm. You should be able to use a wrist cuff on either wrist, too.

From my experience, and what I've read, Omron makes the most accurate BP monitors. If you are using an arm cuff, you should measure your arm to make sure that the cuff will be large/long enough for you. The directions in/on the box, and product description on the website should state the length of the cuff that comes with the monitor. Sometimes you have to buy a separate cuff if you need a larger or smaller one.